The present invention relates in general to a method of recovering pure aromatic substances of a mixture of hydrocarbons which contains both aromatic and an arbitrary amount of non-aromatic substances. In particular, this invention relates to a method of regaining such aromatics by extractive distillation using N-substituted morpholines as selective solvent, the extraction stage being provided with a preliminary after-treatment distilling means.
For separating aromatic substances from mixtures of hydrocarbons by means of extractive distillation with N-substituted morpholines with or without addition of water to serve as selective solvents, is known from prior art. For example, in German Pat. Nos. 1,568,940 and 2,040,025, two methods of this kind have been described which have found large-scale application in practice. It is also conventional, particularly when an extractive distillation stage is used, to subject the charged product, namely the mixture of hydrocarbons, to a preliminary distillation before it is applied to the extraction stage. Normally, the extraction stage consists of one or more extraction columns connected to an output column in which the solvent is separated from the extracted product containing the aromatic substances by distillation. The preliminary distillation serves for separating undesired substances from the charged product before its feeding in the extraction stage. In the case when the charged-in product contains a plurality of aromatics, such as for example benzene, toluene and xylene, the extracted product freed from solving agent must undergo in the additional distillation means (after-treatment distillation) a treatment for decomposing the extract from the aforementioned fractions.
In spite of the fact that the methods described in the aforementioned patent literature have found broad application for recovering aromatics, attempts are still being made to improve such prior-art methods, particularly to reduce the consumption of energy in said methods.